If it weren't for a reddish eye, my guess would be constipation. The symptoms involve a string of feces trails behind the fish, the abdomen swells, the fish has little interest in food, and the fish is sluggish or resting on the bottum. No symptoms of disease are present.
The cure...don't feed the fish for 2 or 3 days.
But with the reddening of the eye that suggests a secondary bacterial infection. That would need to be treated in a seperate hospital tank, but if that's what it is then it may be to far gone to save since he is lethargic.